In known digital audio signal watermarking the audio quality suffers from degradation with each watermark embedding-and-removal step.
One of the dominant approaches for watermarking of multimedia content is called quantisation index modulation denoted QIM, see e.g. B. Chen, G. W. Wornell, “Quantization Index Modulation: A Class of Provably Good Methods for Digital Watermarking and Information Embedding”, IEEE Transaction on Information Theory, vol. 47(4), pp. 1423-1443, May 2001, or J. J. Eggers, J. K. Su, B. Girod, “A Blind Watermarking Scheme Based on Structured Codebooks”, Proc. of the IEE Colloquium on Secure Images and Image Authentication, pp. 1-6, 10 Apr. 2000, London, GB.
With QIM it is possible to achieve a very high data rate, and the capacity of the watermark transmission is mostly independent of the characteristics of the original audio signal.
In QIM as described by B. Chen and G. W. Wornell and mentioned above, an input value x is mapped by quantisation to a discrete output value y=Qm(x), whereby for each watermark message m a different quantiser Qm is chosen. Therefore the detector can in turn try all possible quantisers and detect the watermark message by finding the quantiser with the smallest quantisation error.
J. J. Eggers et al. mentioned above have proposed an extension to QIM in order to achieve better capacity in specific watermark channels: in this α-QIM all input values x are linearly shifted towards the reference value (i.e. towards the centroid of the quantiser) with a constant factor. The watermarked output value y can be considered as being computed by y=Qm(x)+α(x−Qm(x)).